The present invention generally relates to a dental post and core system for endodontically-treated teeth having a flexible, inelastic post. More specifically, this invention relates to an improved method of restoring endodontically-treated teeth.
Dental post and core systems are widely utilized to restore endodontically-treated teeth. Post and core restorations are routinely used to create an adequate foundation for the final restorative step, which may be a crown, onlay, or a fixed partial denture abutment. Generally, a post is provided for retention and lateral stability of the restoration. The core provides support for the crown. Two general types of post and core systems are known in the art: "active or screw-in" type systems and "passive" type systems. Active post and core systems mechanically engage the walls of the root canal and tooth dentin. Passive post and core systems are bonded in a reformed root canal utilizing cements and the like.
Two major problems are encountered when restoring an endodontically-treated tooth. Firstly, the tooth is more susceptible to fracture, and secondly, there is generally less coronal structure with which to work. The greater susceptibility of a tooth to fracture after endodontia may result from the tooth being more brittle. However, studies of the changing mechanical properties of pulpless teeth do not generally support this theory equating dryness with reduced mechanical strength. It appears that the greater susceptibility for fracture in an endodontically-treated tooth results from mechanical weakening of the tooth during root canal therapy and refinement of the canal. Improvements in restoration techniques that reduce mechanical weakening are therefore desirous.
An endodontically-treated tooth is generally severely compromised either due to trauma or neglect. Thus, traumatic fractures, removal of old restorations and carious tissue, and preparation of root canal access may not leave enough tooth to maintain the "dome effect" of the tooth or to retain a crown.
The stress concentrations in a tooth resulting from the prior art rigid post and core systems also play a vital role in tooth fracture. Stress concentrations can be impacted through system design and/or restoration techniques. Various studies and investigations into the susceptibility of endodontically-treated teeth to fracture and the contribution of prior art rigid dental post and core systems to such fracture have been conducted. A Comparison of Intracanal Stresses in a Post-Restored Tooth Utilizing the Finite Element Method, Cailleteau, Johnny G., Rieger, Monty R. and Akin, J. Ed, Journal of Endodontics, Vol. 18, No. 11, November 1992, pp. 540-544, reports that placement of a rigid post within a tooth alters the pattern of stress along the root canal as compared with an intact tooth. Instead of strengthening the tooth the post stiffens the coronal posted section and shifts the flexure point apically. The effect of this stiffening causes the nonposted apical portion of the tooth to deform at the post apex, resulting in a stress increase in that portion of the canal wall. Also, the cyclic loading and unloading of an incisor during mastication requires consideration of fatigue failure. Since the maximum bending stresses occur in connection with the apex of the post, any inclusions or defects within the wall of the dentin near the apical end of the post would create stress concentrations that increase the risk of a fatigue crack formation. Defects and microfractures introduced during endodontic treatment and post access preparation could become areas contributing to stress concentrations. Studies have also shown that more intact tooth structure provides better resistance to fracture than a metallic post. There is also evidence that stresses in the tooth tend to increase as the post diameter increases.
A flexible post eliminates these problems. A post and core system utilizing a flexible post shifts the stress concentrations coronally, eliminates the introduction of defects during post access preparation and post placement, and leaves more intact tooth.
The main function of a post is to provide retention to the core. Relieved of its expectation to facilitate resistence to tooth fracture, the post can be designed to optimize its retentive properties. Several factors govern the retentiveness of endodontic posts. The shape of the post and its length are among the essential factors. Tapered dowels have been found to be significantly less retentive than parallel-sided posts. A serrated 5.5 mm parallel-sided dowel was found more retentive than an 8 mm tapered post. Tapered posts provide high shoulder stresses but an undesirable wedging effect. The wedging effect results in part from the prior art placement of a rigid post in a naturally curved and varying diameter root canal. Active posts are very retentive, but may impose too much stress on the tooth, especially compromised teeth. Thus it appears that a passive, serrated, parallel-sided post is a preferred structure for dental post and core systems. A flexible, passive, serrated parallel-side post provides the previously-mentioned advantages in preventing tooth fracture and additionally permits the post to extend for a greater length into the root canal for improved retention.
In addition to post shape and length adequate retention is a function of cementing mechanisms. Various cementing medium have been studied. Utilization of low viscosity resin cement in combination with smear layer removal can be considered a universal post cementation technique. In addition to good retention, this cementing technique offers the benefits of a cement with very little resistance to post insertion, thereby minimizing stresses applied to tooth structure during cementation. Generally, however, the invention of the present disclosure is not limited by the cementing process used.
An elastic, wire pin having a plurality of flexible, radially-extending fins along its length is disclosed in German Patent No. DE 3643-219 to Weisskircher. While providing some advantages over the prior art rigid post, the "high degree of elasticity" of the Weisskircher pin will cause it to try and retain its initial shape in the canal. During and after placement, flexing of the pin will cause the apical end of the pin to lay against the wall of the root canal. Stress concentrations in the tooth as known for rigid posts will thereby be induced. A pin formed from wire also has low retention characteristics and tends to rotate within the root canal. Radial fins are utilized in the Weisskircher disclosure to resist rotation of the wire pin. However, these radial fins may become further sources of stress concentrations and fatigue failure as the wire pin rotates. No prior art known to the present Applicants discloses or suggests a post in a dental post and core system that is flexible and inelastic, i.e. that conforms to the shape of the root canal to eliminate the stress concentrations that facilitate tooth fracture.
U.S. Pat. No. 4,778,389 to Salvo discloses a dental post construction to eliminate lateral stress in a tooth wherein a rigid, split post is formed by parallel sections joined at a marginal top portion of the post head.
U.S. Pat. No. 5,073,112 to Weil discloses a dental post having an active portion and a passive portion.
U.S. Pat. No. 5,074,792 to Bernadat discloses a passive post and core system comprising a rigid peg disposed in a porous sheath formed of high-strength filaments.
International Search Publication No. WO 91/07142 (PCT/FR90/00831) to Reynaud et al. discloses a dental post and core system having a post formed from equally-tensioned fibers of composite material.
U.S. Pat. No. 4,936,776 to Kwiatkowski discloses a translucent post and core structure to minimize gingival discoloration adjacent a dental restoration.
U.S. Pat. No. 3,949,476 to Kahn discloses a "direct" method of restoring an abraded or broken tooth.
Swiss Patent No. 1,457,914 to Stomatology Research Institute discloses a method of making a pin stump insert.
West German Patent No. 1,541,209 to Kurer discloses the now-conventional threaded, screw-in type active post.
Currently-marketed dental post and core systems such as the FLEXI-POST, the DENTATUS POST, the RADIX POST and the BRASSELEAR screw posts all advocate screwing threaded rigid posts into straight paths machined into the tooth dentin. The present day posts are also generally formed from rigid metals such as steel, titanium and other alloys which do not flex in the same manner as a natural tooth. This differential in flexibility between the natural tooth and the post may cause tooth fracture when the restored tooth is stressed during mastication or from trauma. Cast post are subject to these same limitations and require an additional laboratory fee and an additional visit to the dentist to complete the procedure.
A means to quickly and easily identify the components of a post and core system is also needed in the prior art. Presently, there is either no color coding of post and core systems or the color identification consists of an inconspicuos dot of color. Bright color identification of post and core systems would significantly advance the art. The lack of a color protocol in the prior art creates confusion, eye strain and a sloppy work environment. The inability to readily identify each post and core by sight creates problems before, during and after the procedure is completed. Firstly, before the procedure is initiated the dentist and staff must select the post and core and isolate it from others that may be very close in size. During the procedure the dentist must carefully avoid confusing the selected post and core. After the procedure the used and unused devices must be readily identified for contamination control. Further, a post and core system installed by one dentist may later require an emergency or other procedure by a different dentist in a completely different part of the world. Color-coded identification would eliminate uncertainty and guesswork.
The post and core system of the present invention overcomes all of these limitations of the prior art.